Transcript of Polyethylene Composite The Novel Application of Asphalt in ...
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The Novel Application of Asphalt in Cross-Linked Polyethylene
Composite Yi Liao
State Key Laboratory of Polymer Materials Engineering,polymer
research institute of Sichuan university Shuangxin Lai
State Key Laboratory of Polymer Materials Engineering,polymer
research institute of Sichuan university Shuangqiao
Yang
State Key Laboratory of Polymer Materials Engineering,polymer
research institute of Sichuan university Jinjing Liu
state key laboratory of special functional waterproof materials
shibing bai ( baishibing@scu.edu.cn )
State Key Laboratory of Polymer Materials Engineering,Polymer
Research Institute of Sichuan Universiry
https://orcid.org/0000-0001-8798-8768
Research Article
Posted Date: May 11th, 2021
DOI: https://doi.org/10.21203/rs.3.rs-482238/v1
License: This work is licensed under a Creative Commons Attribution
4.0 International License. Read Full License
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Abstract The cross-linked polyethylene (XLPE) cables are widely
used as an important basic material for power transportation.
However, the cross-linked network structure caused challenge in the
recycling of waste XLPE, which usually be treated by incineration
and landlling. In this research, XLPE was de-crosslinked via
solid-state shear milling (S3M) and the asphalt was used as
plasticizer during the thermoplastic processing. To deeply
understand the inuence of asphalt on the matrix, the compatibility,
dispersion, and rheological properties of composites were
characterized. Due to the good compatibility between de-
crosslinked XLPE and asphalt, the viscosity of the composites
decreased signicantly. Some sea-island structure also formed in
composites, which increased the toughness of the composites and the
elongation at break reach as high as 322%. The use of asphalt to
achieve the processing performance of de-crosslinked XLPE powder
was highly effective and composites can be processed by ordinary
polymer processing equipment. Furthermore, the prepared composites
show potential application in the eld of waterproong which could
recycle of waste XLPE cables in large-scale.
1. Introduction For cross-linked polyethylene (XLPE), the van der
Waals forces between polyethylene molecules are replaced by
chemical bonds[1], which form a three-dimensional network structure
and making it dicult for the molecules to slip. This network
structure effectively improves the mechanical properties, heat
resistance, environmental stress cracking resistance, and chemical
resistance[2]. The XLPE has a wide range of applications such as
cables, conveying pipes, and heat-shrinkable products[3]. The
peroxide cross-linking, irradiation cross-linking, and silane
cross-linking are three main kinds of cross-linking methods in the
industry. The rst two of crosslinking methods used initiators to
achieve cross-linking purposes, and the last one uses physical
methods[4–6]. With the large-scale upgrade of communication and
power grid facilities, a large amount of XLPE cable scrap was
generated. But as an insoluble and infusible thermosetting
material, how to recycle XLPE cables has always been a serious
problem. Conventional direct incineration and landll caused the
leakage of waste liquid, the emission of waste gas, and pollute the
land[7, 8]. They are not in line with the current concept of
sustainable green development[9]. However, the green, high-quality,
and ecient recycling of XLPE waste is still a huge challenge.
According to literature, the recycling methods of XLPE include
thermal shear plasticization recycling[10], powdered ller
recycling, supercritical uid recycling, and ultrasonic-assisted
recycling[11–13]. Above technology faced with defects of poor
mechanical properties of materials, large equipment investment and
strict process requirements[14–16]. Based on the pursuit of simpler
and more ecient XLPE recycling technology, a self-developed
solid-state shear milling (S3M) technology was used to recycle the
material[17–20]. S3M equipment is based on traditional Chinese
stone pan mill, which not only can crush and mix, but also act as a
mechanochemical reactor. With the unique three-dimensional shear
structure, the materials in the equipment are subjected to
shearing, squeezing, and stretching stress elds. The motion
trajectory of material is a spiral line, which has a long path
enables the equipment to realize the
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functions of crushing, dispersing, mixing, etc[17]. This technology
has been reported to destroy the cross- linked bond of silane
cross-linked polyethylene (Si-XLPE) and achieved the fracture
between some -C-Si- and -O-Si-O- bonds[21], resulting in
thermoplastic processable polyethylene. To further realize the
value- added application of XLPE powder, it is important to improve
the processing performance of composites.
Asphalt is the residue of coking or crude oil distillation and it
is mainly composed of alkane compounds composed of carbon and
hydrogen[22]. Due to the characteristics of waterproof,
moisture-proof, and corrosion-proof, asphalt is often used in the
eld of waterproong and paving roads. Nowadays, bituminous
waterproong membrane modied by rubber powder integrated bonding,
sealing, and waterproong and has gradually replaced the traditional
waterproong membrane [23–25]. Due to the presence of small relative
molecular weight alkanes, the asphalt not only has good
thermoplastic uidity but also has good compatibility with
polyethylene.
This study shown the novel application of asphalt in waste peroxide
cross-linked polyethylene (WP-XLPE) composite. In the rst step,
WP-XLPE powder was prepared by S3M technology. Through the strong
shear force, the cross-linked structures of WP-XLPE materials were
destroyed. Then, asphalt added as a plasticizer to give the powder
excellent processing performance. From the results of SEM and DMA,
it can be obtained that the asphalt and WP-XLPE have good
compatibility and uniform dispersion. The rheological test shows
that the addition of asphalt greatly improved the ability of
processing in WP-XLPE thermoplastic extrusion. The composite sheets
can be successfully extruded by a screw. It is expected that such a
sheet has potential to be used as waterproof material.
2. Experimental Protocols
2.1 Materials The waste peroxide cross-linked polyethylene cable
mainly includes three parts: conductor shielding layer, insulating
layer, and insulating shielding layer, provided by TBEA Deyang
Cable Co., Ltd. (Sichuan China), which consisted of 75% P-XLPE and
25% EVA.
90# asphalt: the label is determined by the penetration degree, and
its basic performance is shown in Table. 1, provided by Beijing
Oriental Yuhong Co., Ltd.
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Asphalt 90
Ductility/10 (cm) > 20
Solubility (%) > 99.9
2.2 Preparation of asphalt/WP-XLPE composites The preparation
scheme of asphalt/WP-XLPE composites is shown in Figure.1. The
WP-XLPE cable was crushed into 2–3 cm particles by plastic crusher
and then fed into the solid-state shear milling (S3M) equipment to
prepare the WP-XLPE powder with 10 milling cycles of 50 rpm. In
this milling process, the temperature is controlled by the cooling
water. This instrument used the relative movement between two
milling pans with grooves and ridges to produce a huge shearing
force and make the material spiral outward from the center of the
pan at a rotation speed.
The WP-XLPE powder was mixed with asphalt of different mass ratios
through an internal mixer at a speed of 50rpm and under the
temperature of 180. 10 minutes later, the material was taken out
and crushed. Then the composite material fragments were made into
sheets in two ways: the rst one, under 180 and 10MPa, hot pressing
for ten minutes, then cold pressing under the same pressure for 10
minutes; the second one, extruding with a screw extruder, the
temperatures of barrels were 150, 175, 180, 175 and the screw was
maintained at a speed of 100 rpm.
2.3 Characterization
2.3.1 Fourier Transform Infrared Spectroscopy (FT-IR) A Nicolet
6700 FT-IR spectrometer was utilized to acquire the molecule
structure and functional groups of the samples. The spectra were
recorded from 4,000 cm− 1 to 400 cm− 1 at a resolution of 4 cm− 1
over 32 scans.
2.3.2 X-ray Photoelectron Spectroscopy (XPS) An X-ray photoelectron
spectrometer was used to identify all the elements present in the
composite material through an accelerating voltage of 15 kV and an
emission current of 10 mA.
2.3.3 Dynamic Mechanical Analysis (DMA) A DMA of TA instruments
(model Q850) in three-point bending mode was utilized to study the
dynamic mechanical properties of specimens. Dynamic loss (tan δ)
was determined at a frequency of 1 Hz and a
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heating rate of 3*min− 1as a function of temperature in the range
of -100 to 100.
2.3.4 Scanning Electron Microscope (SEM) All asphalt/WP-XLPE
composites of different components were brittle fractured in liquid
nitrogen and then immersed in n-hexane to etch away the asphalt.
Observed the morphology of the cross-section with an Inspect (FEI)
scanning electron microscopy instrument to analyze the polymer
morphology and the distribution of asphalt in the matrix. All
samples were sputter-coated with gold.
2.3.5 Differential Scanning Calorimetry (DSC) Using the TA Q20
differential scanning calorimeter to test the melting and
crystallization behavior of composite samples, with nitrogen
atmosphere protection to prevent the change of asphalt composition.
Recorded the DSC curve, with the temperature range of -70°C to
200°C, at a heating rate of 10°C* min− 1. In this temperature
range, the sample has undergone a process from heating to
cooling
2.3.6 Thermogravimetric Analysis (TGA) The TGA-Q50 from TA
Instruments was used to study the thermal stability of composite
asphalt materials, using a heating rate of 10°C* min− 1 in nitrogen
atmosphere from 25°C to 700°C.
2.3.7 High-Pressure Capillary Rheometer The high-pressure capillary
rheological analysis was utilized to test the shear ow behavior of
different composition materials. The experimental parameters were
set as follows: L/D = 20/2,180°C, shear rate range of 50 ~ 1500 s−
1.
2.3.8 Torque Rheometer The torque rheometer was utilized to test
the torque rheological properties of materials with different
compositions. About 50g of the sample was added to the rheometer to
melt and mix at 180°C, and the motor speed was set at 50rpm/min for
10min. During the experiment, the torque curve was recorded by the
computer.
2.3.9 Mechanical Properties The universal tensile testing machine
was utilized to test tensile strength and elongation at break. The
experimental samples were subjected to conventional tensile tests
according to the test standard ASTM D412, with a tensile speed of
50 mm/min.
3. Results And Discussion
3.1 Changes in chemical bonds and groups of materials To explore
the relationship between the compatibility of the two in the
composite material and the chemical structure, the rubber powder,
asphalt, and composite materials were characterized by FTIR.
The
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analysis results of WP-XLPE with different milling cycles in
Figure.2(a) show that they all have common characteristic bands:
there are three strong absorption peaks at 2920cm− 1, 1470cm− 1,
and 722cm− 1, which belong to the expansion, bending, and swing
vibrations of C-H, all of which are the infrared characteristic
absorption peaks of polyethylene. And 722cm− 1 belongs to the
absorption peak of amorphous PE[7], indicating that the WP-XLPE
after S3M treatment has a de-crosslinked part. It is worth noting
that as the number of milling cycles increases, there are obvious
characteristic absorption peaks at 1740cm− 1 and 1240cm− 1, which
are carbonyl (-C = O) and C-O stretching vibrations, respectively.
The reason is that the three-dimensional network structure of XLPE
is destroyed by the strong force of the S3M equipment, and a large
number of free radicals are generated, which further react with
oxygen in the air and introduce oxygen-containing groups[26]. As
shown in Figure. 2(b), the composition of asphalt is very complex.
The FTIR of asphalt shows strong absorption vibration peaks at 650
~ 1700cm− 1 and 2700 ~ 3000cm− 1, and the absorption peak at
3430cm− 1 is produced by the associated hydroxyl groups in water
molecules. Asphalt is a mixture of saturated aromatic hydrocarbons
and compounds, alkyl hydrocarbons, and heteroatom derivatives such
as oxygen and sulfur[27]. Therefore, the internal molecular
structure and functional groups are very complicated. As shown in
Figure.2(b), the asphalt- modied WP-XLPE composite material has no
new absorption peak, indicating that there is no obvious chemical
reaction between the two components, and the modication mechanism
is mainly physical.
X-ray photoelectron spectroscopy can obtain chemical bond
composition information in the range of 2- 10nm on the surface of
the material. From the XPS spectrum of the WP-XLPE cable before and
after milling in the gure.3, the results consistent with FTIR can
be obtained. In the high-resolution spectra of C and O, it can be
seen that the powder after milling has a new peak at the position
corresponding to the carbonyl group[28], indicating that when the
powder is decomposed by force, there is indeed a process of
reacting with oxygen to form a C = O bond. This is a part of the
methylene groups deformed and broken under the action of
considerable mechanical stresses by S3M technology, and oxidized
into new carbonyl groups[29, 30]. However, the EVA in the cable
brings natural hydroxyl groups to the WP-XLPE powder, which can
also be seen in the results[31]. This shows that the S3D equipment
developed by our research group is not simply a grinding process
but a solid-phase mechanochemical reactor. In this process, the
thermosetting cross-linked molecular structure is destroyed, and
the chemical bonds are partially reorganized.
Figure.3 also shows the C and O high-resolution spectra of asphalt
modied WP-XLPE composites with different contents. Compared with
the pure WP-XLPE without asphalt, the content of the C = O bond has
increased from 39.8–51%, which indicates that the free radicals
generated during the milling process are continuously oxidized to
produce carbonyl groups during the thermoplastic processing. Since
there was no chemical bond breaking or formation during the asphalt
modication process, no new peaks appeared in the XPS results.
Asphalt plays a role as a plasticizer that weakens intermolecular
forces and increases free volume in composite materials. This is
also mutually corroborated by the absence of new absorption peaks
in the FTIR spectrum.
3.2 Compatibility of Asphalt and WP-XLPE
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The dynamic mechanical properties are shown in Figure.4. The result
of DMA reects the good compatibility of asphalt and WP-XLPE. The α
and β transition peaks of different asphalt content are in the same
temperature range, indicating that the compatibility between the
two phases is good; in other words, the addition of asphalt does
not affect the tan δ of WP-XLPE. The transition area of adding
asphalt becomes narrower, which also shows that the distribution
between the two phases is uniform[32]. The increase in peak
intensity, mainly contributed by the amorphous phase, is due to the
easier movement of small molecules in the asphalt component, which
makes the segment movement more frictional[33]. The movement of the
relaxation temperature to high temperature indicates that the
amorphous WP-XLPE is arranged more regularly under the action of
asphalt, and the structure density is improved, which makes up for
the disordered structure caused by the solid phase
mechanochemistry.
The storage modulus reects the rigidity of the material. As shown
in the gure.4(b), the content of asphalt has a signicant effect on
it. When the temperature is below zero, the chain segment is frozen
and yet to move, but the lower molecular weight asphalt can better
transfer the force received to the rigid particles, thereby
increasing the storage modulus of the material. In addition, as the
temperature increases, the matrix begins to soften, and with
relaxation, the storage modulus decreases. Contrary to the elastic
modulus, the loss modulus is a measure of the energy loss when the
composite material is deformed to reect the toughness of the
material. The loss modulus peak around − 25 in the gure.4(c)
indicates that the asphalt makes the amorphous part of WP-XLPE more
regular and hinders the movement of the chain segment. There is a
rule that the higher the content, the more obvious the
restriction
3.3 Distribution of Asphalt and WP-XLPE To carefully observe the
phase morphology of the two-phase distribution, the cross-section
of the composite material was analyzed by SEM. As shown in
Figure.5, the section of the pure XLPE without asphalt is not only
randomly scattered with particles, but also has many scaly
protrusions; on the contrary, the section of the composite with
asphalt is very at, with no trace of ller at all. It can be
concluded that the two-phase dispersion is very uniform, and there
is no phase separation or incompatibility. Actually, WP-XLPE cable
is a complex mixture, in which the inorganic ller hinders the ow of
the melt during the melting process. The section without asphalt
can nd that the distribution of these llers is very uneven, which
affects the performance of the material. The addition of asphalt
wraps these llers and makes the llers more smoothly distributed in
the matrix. Even in the cross-section, its existence cannot be
seen, showing a very smooth shape.
Observing the etched section in Figure.6, you can see the circular
or deep groove-like voids left by the asphalt after being etched,
which shows that the asphalt has a dispersed state and a continuous
state. The distribution between asphalt and WP-XLPE has both a
typical island-like structure surrounded by asphalt and a
sheet-like structure after melting, indicating that the 3D network
structure is destroyed under the action of solid-state
mechanochemistry[34].
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Under the effect of S3M technology, WP-XLPE is divided into two
parts: fusible polyethylene and non- melting cross-linked network.
It brings about two different behaviors under interaction with
asphalt: the movable part of the chain formed under shear is melted
and mixed with asphalt to form an interlocking continuous layered
structure; the infusible part is wrapped in asphalt to form an
island structure. The asphalt here is like a plasticizer,
connecting the three parts of amorphous, crystalline, and
cross-linked. It makes the movement of the molecular chain more
smoothly at high temperatures and more regularly during
thermoplastic processing. In addition, the sea-island structure
provides toughness to the material.
3.4 Thermal Performance As shown in Figure.7(a, b), with the
addition of asphalt, the processing temperature of the material is
signicantly reduced. There is also a trend that the more the amount
added, the more obvious the melting temperature and crystallinity
will decrease. The content of asphalt destroys the crystal lattice
arrangement of PE and affects the crystallization process of its
material, causing the formation of imperfect crystal, resulting in
a decrease in melting temperature and crystallinity[35]. During the
cooling crystallization process, the endothermic peak shifts to low
temperature with the addition of asphalt. The reason is that the
volume of molecular chain movement increases, so that
crystallization can be achieved at lower temperatures[36]. In
addition, this also reects that the addition of asphalt does act as
a plasticizer, which makes the molecular chain move easily and
contributes to good thermoplastic processing performance.
As far as the thermal stability in Figure.7(c) is concerned, the
thermal weight loss of materials mainly includes two stages: the
rst stage is 250–400, which is mainly the thermal degradation of
EVA and asphalt; the second stage is 400–500, which is mainly the
thermal degradation of the WP-XLPE matrix. Obviously, the whole
process is mainly based on the second stage. In the rst stage,
because asphalt is a low-molecular mixture with an average
molecular weight of about 1,000, as the temperature rises, the
asphalt in the composite material begins to decompose rst. The
low-temperature decomposition temperature is relatively stable, but
the mass lost at this temperature increases with the increase of
the asphalt content. In the second stage, the decomposition
temperature tends to move to high temperature with the addition of
asphalt, indicating that the thermal stability of the composite
material has become better[37].
3.5 Processing Performance In this paper, a high-pressure capillary
rheometer was used to test the processing performance of composite
materials with different asphalt additions. It can be seen from
Figure.8(a) that this is a typical shear thinning situation, and as
the asphalt content increases, the shear viscosity of the material
decreases, especially in the low-frequency range[38]. The same
result can also be obtained in the torque rheology curve in
Figure.8(b). When the asphalt addition amount is 20%, the torque
drops suddenly, indicating the melt viscosity is low and the
thermoplastic processing is easier. This shows that when the
material is in the molten state, asphalt as a plasticizer moves
between the cross-linked phase and the amorphous phase[39]. At the
same time, it plays a role in reducing the entanglement and
friction between
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the molten phase and the infused phase to make the melt ow
smoothly. The plasticization mechanism is called the volume effect,
because of the addition of asphalt increasing the distance between
polymer molecules and decreasing the force between the
molecules[40, 41]. The weakening of van der Waals force brings
about a drop in melt viscosity. The existence of asphalt makes up
for the shortcomings of dicult thermoplastic processing of pure
WP-XLPE powder.
3.6 Mechanical Performance The tensile data of pure WP-XLPE in the
gure.9 shows that after ten times of milling, the waste cable
material has regained the thermoplastic processability and the
recycled material also has better mechanical performance. This is
consistent with the statement in a large number of documents: S3M
technology has the function of de-crosslinking and provides ideas
for recycling thermoset plastics. For asphalt-modied WP-XLPE
composite materials, the addition of asphalt not only improves the
processing performance of WP-XLPE but also affects the mechanical
properties of the material. As the asphalt content increases,
Young's modulus and tensile strength decrease to a certain extent,
but the fracture energy and elongation at break have a peak. When
the asphalt content is 10%, the elongation can reach 322%, and the
tensile strength remains above 10MPa.The reason for the above
phenomenon are as follows: asphalt has the effect of weakening the
intermolecular force of WP-XLPE, increasing the uidity of the
molecular chain, and reducing the crystallinity of the molecular
chain, which leads to a decrease in strength and an increase in
toughness. This is no different from ordinary plasticizers, but the
similar chemical composition and structure lead to good
compatibility with the fusible phase. Finally, it can be directly
processed by screw extrusion to obtain a sheet with a stable
structure and smooth surface as shown in Figure.9(c).
4. Conclusion Although the powder processed by S3M technology can
be processed by thermoplastic, the ller and cross-linked part in
the material hinder the movement of the molecular chain, which
makes the screw extrusion molding dicult and the sample surface
rough. The purpose of this research is to improve the melt uidity
of WP-XLPE. The addition of asphalt can solve the dispersion
relationship among ller, cross- linked part, and decross-linked
part. Due to the principle of like dissolves like, the two have
good compatibility. Furthermore, the role of asphalt is also
highlighted: increasing the intermolecular gap, promoting molecular
movement, and reducing melt viscosity, which effectively reduced
the diculty of thermoplastic processing. The asphalt with good
uidity also entrains the ller in WP-XLPE to make it more evenly
dispersed in the matrix. In addition, the asphalt and the
non-melting cross-linked part form a sea-island structure, which
enhances the toughness of the material to a certain extent. When
the asphalt content is 10%, the elongation can reach 322%, and the
tensile strength remains above 10MPa. Different from the complex
process of traditional waterproong membranes, this material can be
directly prepared into continuous membranes by screw extrusion. At
last, the use of WP-XLPE cable material as the matrix also provides
a new direction for large-scale recycling of waste, bringing
considerable economic and social benets.
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Declarations
Acknowledgment This work is nanced by the National Key Research and
Development Program of China (2019YFC1908200), the National Natural
Science Foundation of China (51861165203), and the Program for
Featured Directions of Engineering Multidisciplines of Sichuan
University (No: 2020SCUNG203).
Declarations Funding: This work is supported by the National Key
Research and Development Program of China (2019YFC1908200), the
National Natural Science Foundation of China (51861165203), and the
Program for Featured Directions of Engineering Multidisciplines of
Sichuan University (No: 2020SCUNG203).
Conicts of interest/Competing interests: The authors declared no
conict of interest.
Availability of data and material: The data are available from the
corresponding author on reasonable request.
Code availability: Not applicable
Authors' contributions: All authors contributed to the study
conception and design. Material preparation, data collection and
analysis were performed by Yi Liao. And all authors commented on
previous versions of the manuscript. All authors read and approved
the nal manuscript.
Ethics approval: This research was not involved with any human or
animal experiment. And ethics approval was not required for this
research.
Consent to participate: All authors agree to submit.
Page 11/20
Consent for publication: All authors agree to publish.
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Figures
Page 15/20
Figure 2
FTIR spectra of WP-XLPE with different milling cycles (a), asphalt,
and the composite (b)
Page 16/20
Figure 3
Comparison of XPS spectra of WP-XLPE before and after S3M, and the
composite with different asphalt content: XPS spectra for C1s (a),
O1s(b)
Page 17/20
Figure 4
Page 18/20
Figure 5
SEM images of the fracture surface of WP-XLPE/asphalt composites:
without asphalt (a),10wt% asphalt (b),20wt% asphalt (c),30wt%
asphalt (d)
Page 19/20
Figure 6
SEM images of the fracture surface after etched of WP-XLPE/asphalt
composites:10wt% asphalt (a),20wt% asphalt (b),30wt% asphalt
(c),40wt% asphalt (d)
Figure 7
The DSC curve (a, b) and TGA curve (c) of composites with different
asphalt content
Page 20/20
Figure 8
Capillary rheological curve (a) and the torque variation (b) of
composites with different asphalt content
Figure 9